In order to improve system performance, for example by improving the coverage of high data rates, improving the cell-edge throughput and/or increasing system throughput, Coordinated Multipoint (CoMP) transmission and/or reception may be used in a wireless communications system or radio access network. In the wireless communications system or cellular radio communications system wireless devices and/or user equipments, also known as mobile terminals and/or wireless terminals, communicate via a Radio Access Network (RAN) with one or more core networks. The user equipments may be mobile stations or user equipment units such as mobile telephones, also known as “cellular” telephones, and laptops with wireless capability, e.g., mobile termination, and may thus be, for example, portable, pocket, hand-held, computer-Included, or car-mounted mobile devices which communicate voice and/or data via the radio access network. A wireless device may be any equipment being wirelessly connectable to a RAN for wireless communication.
The radio access network covers a geographical area which is divided into point coverage areas, traditionally denoted cells, with each point coverage area or cell being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called “eNB”, “eNodeB”, “NodeB” or “B node” and which in this document also is referred to as a base station or radio network node. A point coverage area is a geographical area where radio coverage is provided by a point, also referred to as a “transmission point” and/or a “reception point”, which is controlled by the radio base station or radio network node at a base station site or radio network node site. A point coverage area is often also denoted a cell, but the concept of a cell also has architectural implications and the transmission of certain reference signals and system information. More specifically, multiple point coverage areas may jointly form a single logical cell sharing the same physical cell ID. However, in the following the notation of a “cell” is used interchangeably with “point coverage area” to have the meaning of the latter. Moreover, a point, or “transmission point” and/or a “reception point”, corresponds in the present disclosure to a set of antennas covering essentially the same geographical area in a similar manner. Thus, a point might correspond to one of the sectors at a site, e g a base station site, but it may also correspond to a site having one or more antennae all intending to cover a similar geographical area. Often, different points represent different sites. Antennas correspond to different points when they are sufficiently geographically separated and/or have antenna diagrams pointing in sufficiently different directions.
The radio network node communicates over an air interface or radio interface with the user equipments within the range of the radio network node. One radio network node may serve one or more cells via one or more antennas operating on radio frequencies. The cells may be overlaid on each other, e g as macro and pico cells having different coverage areas, or adjacent to each other, e g as so called sector cells where the cells served by the radio network node each cover a section of the total area or range covered by the radio network node. The cells adjacent or overlaid relative to each other may alternatively or additionally be served by different or separate radio network nodes that may be co-located or geographically separated.
The one or more antennas controlled by the radio network node may be located at the site of the radio network node or at antenna sites that may be geographically separated from each other and from the site of the radio network node. There may also be one or more antennas at each antenna site. The one or more antennas at an antenna site may be arranged as an antenna array covering the same geographical area or arranged so that different antennas at the antenna site have different geographical coverage. An antenna array may also be co-located at one antenna site with antennas that have different geographical coverage as compared to the antenna array. In the subsequent discussion an antenna or antenna array covering a certain geographical area is referred to as a point, or transmission and/or reception point, or more specifically for the context of this disclosure as a Transmission Point (TP). In this context multiple transmission points may share the same physical antenna elements, but could use different virtualizations, e.g., different beam directions.
The communications, i e transmission and reception of signals between the radio access network and a user equipment, may be performed over a communication link or communication channel via one or more transmission and/or reception points that may be controlled by the same or different radio network nodes. A signal may thus, for example, be transmitted from multiple antennas by being transmitted via one transmission point from more than one antenna in an antenna array or by being transmitted via more than one transmission point from one antenna at each transmission point. The coupling between a transmitted signal and a corresponding received signal over the communication link may be modelled as an effective channel comprising the radio propagation channel, antenna gains, and any possible antenna virtualizations. Antenna virtualization is obtained by precoding a signal so that it can be transmitted on multiple physical antennae, possibly with different gains and phases. Link adaptation may be used to adapt transmission and reception over the communication link to the radio propagation conditions.
An antenna port is a “virtual” antenna, which is defined by an antenna port-specific reference signal. An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed. The signal corresponding to an antenna port may possibly be transmitted by several physical antennas, which may also be geographically distributed. In other words, an antenna port may be virtualized over one or several transmission points. Conversely, one transmission point may transmit one or several antenna ports.
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
The Long Term Evolution (LTE) standard is currently evolving with enhanced MIMO support. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. A current working assumption in LTE-Advanced, i e 3GPP Release-10, is the support of an eight-layer spatial multiplexing mode with possibly channel dependent precoding. The spatial multiplexing mode is aimed for high data rates in favourable channel conditions. An illustration of the spatial multiplexing mode is provided in FIG. 1. Therein, the transmitted signal, represented by an information carrying symbol vector a is multiplied by an NT×r precoder matrix WNT×r, which serves to distribute the transmit energy in a subspace of the NT-dimensional vector space, corresponding to NT antenna ports. The precoder matrix is typically selected from a codebook of possible precoder matrices, and is typically indicated by means of a Precoder Matrix Indicator (PMI), which together with a Rank Indicator (RI) specifies a unique precoder matrix in the codebook. If the precoder matrix is confined to have orthonormal columns, then the design of the codebook of precoder matrices corresponds to a Grassmannian subspace packing problem. The r symbols in s each are part of a symbol stream, a so-called layer, and r is referred to as the rank or transmission rank. In this way, spatial multiplexing is achieved since multiple symbols can be transmitted simultaneously over the same Resource Element (RE) or Time-Frequency Resource Element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink, and Discrete Fourier Transform (DFT) precoded OFDM in the uplink. The basic LTE physical resource can be seen as a time-frequency grid, as illustrated in FIG. 2, where each time-frequency resource element (TFRE) corresponds to one subcarrier during one OFDM symbol interval, on a particular antenna port. The resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one slot in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two time-consecutive resource blocks represent a resource block pair, which corresponds to the time interval upon which scheduling operates.
The received NR×1 vector yn for a certain resource element on subcarrier n or, worded differently, data RE number n or TFRE number n, assuming no inter-cell interference, is modeled byyn=HnWNT×rsn+en  (1)
where n denotes a transmission occasion in time and frequency, and en is a noise and interference vector obtained as realizations of a random process. The precoder, or precoder matrix, for rank r, WNT×r, can be a wideband precoder, which may be constant over frequency, or frequency selective.
The precoder matrix is often chosen to match the characteristics of the NR×NT MIMO channel Hn, also denoted channel matrix, resulting in so-called channel dependent precoding. When based on User Equipment (UE) feedback, this is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE or wireless device. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE or wireless device, the inter-layer interference is reduced.
In closed-loop precoding, the UE or wireless device transmits, based on channel measurements in the forward link, i e the downlink, recommendations to the radio network node or base station of a suitable precoder to use. A single precoder that is supposed to cover a large bandwidth, so called wideband precoding, may be fed back. It may also be beneficial to match the frequency variations of the channel and instead feed back a frequency-selective precoding report, e.g. several precoders, one per subband. This is an example of the more general case of Channel State Information (CSI) feedback, which also encompasses feeding back other entities or information than precoders to assist the radio network node or base station in subsequent transmissions to the UE or wireless device. Such other information may include Channel Quality Indicators (CQIs) as well as Rank Indicator (RI).
In Release 8 and 9 of LTE the CSI feedback is given in terms of a transmission rank indicator (RI), a precoder matrix indicator (PMI), and channel quality indicator(s) (CQI). The CQI/RI/PMI report can be wideband or frequency selective depending on which reporting mode that is configured. This means that for CSI feedback LTE has adopted an implicit CSI mechanism where a UE does not explicitly report e.g., the complex valued elements of a measured effective channel, but rather the UE recommends a transmission configuration for the measured effective channel. The recommended transmission configuration thus implicitly gives information about the underlying channel state.
The RI corresponds to a recommended number of streams that are to be spatially multiplexed and thus transmitted in parallel over the effective channel. The PMI identifies a recommended precoder (in a codebook) for the transmission, which relates to the spatial characteristics of the effective channel. The CQI represents a recommended transport block size, i.e., code rate. There is thus a relation between a CQI and a Signal to Interference and Noise Ratio (SINR) of the spatial stream(s) over which the transport block is transmitted. Therefore, noise and interference estimates are important quantities when estimating, for example, the CQI, which is typically estimated by the UE or wireless device and used for link adaptation and scheduling decisions at the radio network node or base station side.
The term en in (1) represents noise and interference in a TFRE and is typically characterized in terms of second order statistics such as variance and correlation. The interference can be estimated in several ways. For example it can be estimated as the residual noise and interference on the TFREs of the Cell Specific Reference Signal (CRS), after the known CRS sequence has been pre-subtracted, i.e., after the CRS has been cancelled. An illustration of CRS for Rel-8 of LTE can be seen in FIG. 3.
In LTE Release-10, a new reference symbol sequence was introduced, the Channel State Information Reference Signal (CSI-RS), intended to be used for estimating channel state Information. The CSI-RS provides several advantages over basing the CSI feedback on the CRS which were used, for that purpose, in previous releases. Firstly, the CSI-RS is not used for demodulation of the data signal, and thus does not require the same density. This means that the overhead of the CSI-RS is substantially less as compared to that of CRS. Secondly, CSI-RS provides a much more flexible means to configure CSI feedback measurements: For example, which CSI-RS resource to measure on can be configured in a UE specific manner. Moreover, antenna configurations larger than 4 antennas must resort to CSI-RS for channel measurements, since the CRS is only defined for at most 4 antennas.
A detailed example showing which resource elements within a resource block pair may potentially be occupied by UE-specific RS and CSI-RS is provided in FIG. 4. In this example, the CSI-RS utilizes an orthogonal cover code of length two to overlay two antenna ports on two consecutive REs. As seen, many different CSI-RS patterns are available. For the case of 2 CSI-RS antenna ports, for example, there are 20 different patterns within a subframe. The corresponding number of patterns is 10 and 5 for 4 and 8 CSI-RS antenna ports, respectively.
A CSI-RS resource may be described as the pattern of resource elements on which a particular CSI-RS configuration is transmitted. One way of determining a CSI-RS resource is by a combination of the parameters “resourceConfig”, “subframeConfig”, and “antennaPortsCount”, which may be configured by Radio Resource Control (RRC) signaling.
Related to CSI-RS is the concept of zero-power CSI-RS resources, also known as a muted CSI-RS, that are configured just as regular CSI-RS resources, so that a UE knows that the data transmission is mapped around those resources. The intent of the zero-power CSI-RS resources is to enable the network to mute the transmission on the corresponding resources so as to boost the SINR of a corresponding non-zero power CSI-RS, possibly transmitted in a neighbor cell/transmission point. For Rel-11 of LTE, a special zero-power CSI-RS that a UE is mandated to use for measuring interference plus noise is under discussion. As the name indicates, a UE can assume that the TPs of interest are not transmitting on the muted CSI-RS resource and the received power can therefore be used as a measure of the interference plus noise level. For the purpose of improved interference measurements the agreement in LTE Release 11 is that the network will be able to configure a UE to measure interference on a particular interference Measurement Resource (IMR) that identifies a particular set of TFREs that is to be used for a corresponding interference measurement.
Based on a specified CSI-RS resource, that defines an effective channel for the data transmission, and an interference measurement configuration, e.g. a muted CSI-RS resource, the UE can estimate the effective channel and noise plus interference, and consequently also determine which rank, precoder and transport format to recommend that best match the particular effective channel.
CoMP transmission and reception refers to a system where the transmission and/or reception at multiple, geographically separated antenna sites is coordinated in order to improve system performance. More specifically, CoMP refers to coordination of antenna arrays that have different geographical coverage areas. The coordination between points can either be distributed, by means of direct communication between the different sites, or by means of a central coordinating node. A further coordination possibility is a “floating cluster” where each transmission point is connected to, and coordinates, a certain set of neighbors (e.g. two neighbors). A set of points that perform coordinated transmission and/or reception is referred to as a CoMP coordination cluster, a coordination cluster, or simply as a cluster in the following. In particular, a goal of using CoMP in a wireless communications network is to distribute the user perceived performance more evenly in the network by taking control of the interference in the system, either by reducing the interference and/or by better prediction of the interference. CoMP operation targets many different deployments, including coordination between sites and sectors in cellular macro deployments, as well as different configurations of Heterogeneous deployments, where for instance a macro node coordinates the transmission with pico nodes within the macro coverage area. In FIGS. 5-7 examples of wireless communications network deployments with CoMP coordination clusters comprising three transmission points, denoted TP1, TP2 and TP3 are shown. The term CoMP is sometimes understood to imply that different transmission points have different geographical locations. However, for the purposes of embodiments of this disclosure, the coordinated transmission aspect is relevant also for situations where transmission points involved in coordinated transmission have the same geographical location. For example, multiple transmission points may in this context share the same physical antenna elements, but could use different virtualizations, e.g., different beam directions, as mentioned in the earlier discussion about Transmission points herein. Although CoMP is referred to as an example in the following discussion of this disclosure, it is not to be understood as limiting for the applicability of the teachings herein.
There are many different CoMP transmission schemes that are considered; for example:
Dynamic Point Blanking where multiple transmission points coordinate the transmission so that neighboring transmission points may mute the transmissions on the time-frequency resources (TFREs) that are allocated to UEs that experience significant interference.
Coordinated Beamforming where the TPs coordinate the transmissions in the spatial domain by beamforming the transmission power in such a way that the interference to UEs served by neighboring TPs is suppressed.
Dynamic Point Selection where the data transmission to a UE may switch dynamically (In time and frequency) between different transmission points, so that the transmission points are fully utilized.
Joint Transmission where the signal to a UE is simultaneously transmitted from multiple TPs on the same time/frequency resource. The aim of joint transmission (JT) is to increase the received signal power and/or reduce the received interference, if the cooperating TPs otherwise would serve some other UEs without taking the JT UE into consideration.
A common denominator for the CoMP transmission schemes is that the network needs CSI information not only for the serving TP, but also for the channels linking the neighboring TPs to a terminal or UE. By, for example, configuring a unique CSI-RS resource per TP, a UE can resolve the effective channels for each TP by measurements on the corresponding CSI-RS. Note that the UE is likely unaware of the physical presence of a particular TP, it is only configured to measure on a particular CSI-RS resource, without knowing of any association between the CSI-RS resource and a TP.
Several different types of CoMP feedback are possible. Most alternatives are based on per CSI-RS resource feedback, possibly with CQI aggregation of multiple CSI-RS resources, and also possibly with some sort of co-phasing information between CSI-RS resources. The following is a non-exhaustive list of relevant alternatives (note that a combination of any of these alternatives is also possible):
Per CSI-RS resource feedback corresponds to separate reporting of channel state information (CSI) for each of a set of CSI-RS resources. Such a CSI report may, for example, comprise one or more of a Precoder Matrix Indicator (PMI), Rank Indicator (RI), and/or Channel Quality Indicator (CQI), which represent a recommended configuration for a hypothetical downlink transmission over the same antennas used for the associated CSI-RS, or the RS used for the channel measurement. More generally, the recommended transmission should be mapped to physical antennas in the same way as the reference symbols used for the CSI channel measurement.
Typically there is a one-to-one mapping between a CSI-RS and a TP, in which case per CSI-RS resource feedback corresponds to per-TP feedback; that is, a separate PMI/RI/CQI is reported for each TP. Note that there could be interdependencies between the CSI reports; for example, they could be constrained to have the same RI. Interdependencies between CSI reports have many advantages, such as; reduced search space when the UE computes feedback, reduced feedback overhead, and in the case of reuse of RI there is a reduced need to perform rank override at the eNodeB.
The considered CSI-RS resources are configured by the eNodeB as the CoMP Measurement Set. In the example shown in FIG. 5, different measurement sets may be configured for wireless devices 540 and 550. For example, the measurement set for wireless device 540 may consist of CSI-RS resources transmitted by TP1 and TP2, since these points may be suitable for transmission to device 540. The measurement set for wireless device 550 may instead be configured to consist of CSI-RS resources transmitted by TP2 and TP3. The wireless devices will report CSI information for the transmission points corresponding to their respective measurement sets, thereby enabling the network to e.g. select the most appropriate transmission point for each device.
Aggregate feedback corresponds to a CSI report for a channel that corresponds to an aggregation of multiple CSI-RS. For example, a joint PMI/RI/CQI can be recommended for a joint transmission over all antennas associated with the multiple CSI-RS.
A joint search may however be too computationally demanding for the UE, and a simplified form of aggregation is to evaluate an aggregate CQI which are combined with per CSI-RS resource PMIs, which should typically all be of the same rank corresponding to the aggregated CQI or CQIs. Such a scheme also has the advantage that the aggregated feedback may share much information with a per CSI-RS resource feedback. This is beneficial, because many CoMP transmission schemes require per CSI-RS resource feedback, and to enable eNodeB flexibility in dynamically selecting CoMP scheme, aggregated feedback would typically be transmitted in parallel with per CSI-RS resource feedback. To support coherent joint transmission, such per CSI-RS resource PMIs can be augmented with co-phasing information enabling the eNodeB to rotate the per CSI-RS resource PMIs so that the signals coherently combine at the receiver.
For efficient CoMP or coordinated transmission operation it is equally important to capture appropriate interference assumptions when determining the CQIs as it is to capture the appropriate received desired signal. Within a coordination cluster an eNodeB can to a large extent control which TPs that interfere a UE in any particular TFRE. Hence, there will be multiple interference hypotheses depending on which TPs are transmitting data to other terminals. In other words, the network can thus control the interference seen on a IMR, by for example muting all TPs within a coordination cluster on the associated TFREs, in which case the terminal will effectively measure the Inter CoMP cluster Interference. In the example shown in FIG. 5, this would correspond to muting TP1, TP2 and TP3 in the TFREs associated with the IMR. However, it is essential that an eNodeB can accurately evaluate the performance of a UE given different CoMP transmission hypotheses—otherwise the dynamic coordination becomes meaningless. Thus the system need to be able to track/estimate also different intra-cluster interference levels corresponding to different transmission and blanking hypotheses.
Consider for example a dynamic point blanking scheme, where there are at least two relevant interference hypotheses for a particular UE: in one interference hypothesis the UE sees no interference from the coordinated transmission point; and in the other hypothesis the UE sees interference from the neighbouring point. To enable the network to effectively determine whether or not a TP should be muted, the network may configure the UE to report two, or generally multiple, CSIs corresponding to different interference hypotheses. Continuing the example of FIG. 5, assume that the wireless device 550 is configured to measure CSI from TP3. However, there may potentially be an interfering transmission from TP2, depending on how the network schedules the transmission. Thus, the network may configure the device 550 for measuring the CSI-RS transmitted by TP3 for two interference hypotheses, the first one being that TP2 is silent, and the other one that TP2 is transmitting an interfering signal.
To harvest the gains of introducing coordinated transmission or CoMP feedback it is essential that a radio network node or base station, e g an eNodeB, can accurately predict the performance of a UE or wireless device for various coordinated transmission hypotheses, in order to select an appropriate downlink assignment. To this end, accurate interference measurements at a terminal are a key element for CSI reporting targeting different transmission hypotheses. However, current state of the art solutions for interference measurements are constrained by current standards and/or limitations imposed by UE specific muting of data channels, making accurate interference measurements difficult, in particular for CoMP systems employing dynamic point selection and/or joint transmission, where the transmission point association to a UE varies dynamically in time. Furthermore, it is essential that when the transmission configuration, e g the transmission point association, to the UE varies dynamically in time, the UE is still capable of correctly decoding received transmissions.
Thus, there is a need for improved handling of muting configurations, such as UE specific muting of data channels when different transmission configurations are available for transmitting information carrying signals in a wireless communications system.